Morphological correlates of migratory behavior in the black cutworm (Lepidoptera: Noctuidae)

Male migrant black cutworms, Agrotis ipsilon (Hufnagel), were sampled by pheromone trapping in several locations in the central and northern Corn Belt of North America in the spring of 1985 and 1990. Moths were also sampled in the southern United States at several locations suspected or known to be sources of migrant black cutworms. Forewing length, forewing width, prothoracic width, and body length were measured; and relative darkness of thoracic pubescence was scored for each moth. By comparing the morphology of northern moths (migrants, by definition) with that of southern moths (presumably containing mixtures of migrants and nonmigrants), we hoped to determine whether migrant A. ipsilon could be distinguished morphologically from nonmigrants. Principal component analyses extracted two important axes from the raw data. The first was a general-size axis, and the second was primarily a relative-darkness, or color, axis. All analyses were performed on the factor scores along these axes. There was no difference in mean size of northern and southern moths, nor was there a consistent latitudinal gradient in size between northern locations. There was, however, reduced variation in the migrant populations; i.e., very small and very large moths were underrepresented in the northern compared with the southern populations. Comparisons between northern moths thought to have originated in the Brownsville, Texas, region suggested that migrants tended to have lighter coloration than those in the source population. But light coloration by itself cannot indicate migrant individuals because, in one instance (Columbia, MO 1985), a migrant population was darker than populations from all other locations sampled that year. The data indicate that, at least in the case of males, migrants cannot be distinguished morphologically from nonmigrants. The results are consistent with the hypothesis that A. ipsilon is an obligate migrant and that there is no distinctive migratory phase induced by environmental cues or conditions.